In recent years, photovoltaic batteries or clean energy sources have been attracting attention as next-generation energy sources. The applications ranging from the architecture field to electric/electronic parts have been developed. The structure of the photovoltaic battery unit is based on a structure of a high light transmission material, photovoltaic battery module, filled resin layer, and back sealing sheet. It is integrated into the house roof and used for electric/electronic parts. The thermoplastic resin sheet is used as the back sealing sheet which is used for some items of the structure of photovoltaic battery. With reference to the thermoplastic resin sheet (for photovoltaic batteries), the durability in the natural environment (hydrolysis resistance, weatherability) is strongly required. Furthermore, the improvement in electrical transduction efficiency of sunlight of photovoltaic batteries is also required. Therefore, the reflected light of a back sealing film of photovoltaic batteries is also used so as to be converted into electricity. Additionally, lightweight properties, strength, and processability of batteries are being requested. Japanese Patent Application Laid-Open (JP-A) No. 2002-26354 (page 2, the first column, lines 32 to 39) discloses the use of a base polymer having a large number average molecular weight as a back sheet for photovoltaic batteries with a low specific gravity. However, the resistance to UV rays and screening potency are inferior and thus further improvement is required.
In the case of the photovoltaic battery module used outdoors, a structure in which the photovoltaic battery is formed on the tempered glass board or a metal substrate with a synthetic resin is generally used to ensure reliability by improving the mechanical strength or the environment resistance which does not easily deteriorate under environmental atmosphere. When the modular structure by the laminating method is more specifically described, a structure in which layers of ethylene-vinyl acetate copolymer (hereinafter referred to as “EVA”) sheet/a photovoltaic battery/an EVA sheet/an aluminum foil sandwiched between vinyl fluoride sheets (hereinafter referred to as “aluminum-fluorine composite sheet”) are in this order laminated and heat-crimped on a tempered glass is used.
When the photovoltaic battery is a thin-film photovoltaic cell comprising an amorphous silicon, a photovoltaic battery obtained by directly forming the photovoltaic battery on the tempered glass board, laminating the EVA sheet and the aluminum-fluorine composite sheet, and heat-crimping it is used. It is a known fact that a polyethylene resin, a polyester resin, and a fluorine film are used as a back sealing film for photovoltaic batteries (see JP-A No. 11-261085 (page 2, the 36th line of the first column to the 4th line of the second column) and JP-A No. 11-186575 (the 36th line of the first column of page 2 to the 22nd line of the first column of page 3)).
A polyester film having air bubbles (see Japanese Patent Application Publication (JPB) No. 7-37098 (the 1st line of the first column of page 1 to the 23rd line of the second column of page 3)) is known. However, these films are not used as a back sheet for photovoltaic batteries.
The conventional sheets had the following problems. Biaxial stretched polyester resin sheets which are conventionally used in the field lack in hydrolysis resistance which is most required for the environment resistance, and thus the use thereof in the field has been limited. As for the biaxial stretched polyester resin sheet which is white-colored, the reflectance is improved, however, the sheet lacks in the hydrolysis resistance. Thus, the use thereof in the field has been limited.
Further, the fluorine sheet is excellent in hydrolysis resistance or weatherability. However, it lacks in gas barrier properties and the sheet nerve is weak, which is disadvantageous. The sheet has been thus used by laminating metallic foils such as aluminium to improve barrier properties and to provide the strength of back sealing material. Even if the film in Patent document 1 which is invented to solve these problems is used, peeling from the lamination interface is occurred, which causes troubles when the photovoltaic battery is produced or when it is installed on a roof.
Although the sheet formed by using polyester sheet is relatively inexpensive, there has been a difficulty in heat resistance when exposed to high temperatures (100 to 120° C.).
It could thus be helpful to provide an inexpensive and excellent polyester resin sheet for photovoltaic batteries which enhances the electrical transduction efficiency of photovoltaic batteries by improving the hydrolysis resistance and interfacial-peeling prevention and further enhancing the screening potency as well as a photovoltaic battery formed by using the same from the background of the related art.
Enhancement of the electrical transduction efficiency of photovoltaic batteries by improving the hydrolysis resistance and heat resistance and further enhancing the screening potency has been intensively examined from the background of the related art. We have thus developed polyester resin sheets for photovoltaic batteries which satisfy the specific UV transmittance, relative reflectance, apparent density, optical density, variation in optical density, and number average molecular weight.
That is, we provide, the following:                (1) A polyester resin sheet for photovoltaic batteries, comprising a polyester resin layer formed by using one or more layers having a number average molecular weight of 18500 to 40000 in which at least one or more layers having 5 to 40% by weight of titanium dioxide is formed on the polyester resin layer, wherein a light transmittance at wavelengths of 300 to 350 nm is 0.005 to 10%, a relative reflectance is 80% or more and 105% or less, an apparent density is 1.37 to 1.65 g/cm3, an optical density is 0.55 to 3.50, and the variation in optical density has a fluctuation of within 20% from the center value;        (2) The polyester resin sheet for photovoltaic batteries according to (1), wherein the variation in optical density has a fluctuation of within 15% from the center value;        (3) The polyester resin sheet for photovoltaic batteries according to (1), wherein the total light transmittance of a thermoplastic resin sheet is 0.005 to 25%;        (4) The polyester resin sheet for photovoltaic batteries according to (1), wherein the thickness of the layer having 5 to 40% by weight of titanium dioxide is 7 to 100% of the total thickness of the polyester resin layer;        (5) The polyester resin sheet for photovoltaic batteries according to any one of (1) to (4), wherein elongation retention before and after aging for 3000 hours is 40 to 100% under conditions of a temperature of 85° C. and a humidity of 85% RH;        (6) The polyester resin sheet, for photovoltaic batteries according to any one of (1) to (5), wherein elongation retention before and after aging for 15 hours is 40 to 100% under conditions of a temperature of 140° C.;        (7) The polyester resin sheet for photovoltaic batteries according to any one of (1) to (6), having a gas and moisture vapor barrier layer;        (8) A polyester resin sheet laminate for photovoltaic batteries, wherein at least the polyester resin sheet according to any one of (1) to (7) and a gas and moisture vapor barrier layer are laminated;        (9) The polyester resin sheet laminate for photovoltaic batteries according to any one of (1) to (7), wherein a moisture vapor transmission rate is 0.5 g/(m2·24 hr) or less using 100 μm conversion factor at a temperature of 40° C. and a humidity of 90% RH in the measurement of moisture vapor transmission rate measurement in accordance with JIS-K-7129;        (10) A photovoltaic battery backside protection sheet, wherein the photovoltaic battery backside protection sheet is the polyester resin sheet for photovoltaic batteries or the polyester resin sheet laminate for photovoltaic batteries according to any one of (1) to (9), used for a back sealing material of photovoltaic batteries; and        (11) A photovoltaic battery module, wherein the photovoltaic battery backside protection sheet according to (10) is used.        
There can be provided an inexpensive and excellent thermoplastic resin sheet for photovoltaic batteries which enhances the electrical transduction efficiency of photovoltaic battery by improving the hydrolysis resistance and heat resistance and further enhancing the screening potency, as well as a layered product. The sheet and the layered product can be suitably used for photovoltaic batteries to be used as roofing materials, photovoltaic batteries having flexibility, and electronic parts.